Reduction In Creep Strength Research Articles

Estimation the Remaining Life for Reformer Unit Tubes Furnace Operated Beyond the Design Life

The aim of this work is to study and model creep damage accumulation for remaining safe working life estimation of tube material used in petroleum industry plants operated at a high temperature for long time beyond the design life. Material constants and other data required for life estimation were extracted from the output results of real accelerated creep rupture tests conducted at three test temperatures (700, 725, and 750(oC and at three constant stress applications (120, 130, and 140 MPa). Also, a time-temperature parameter method (Larson-Miller Method) was modified to be applied based on the results of the accelerated creep rupture tests to build life predicted master curve for austenitic stainless steel type 321H. It is found that creep rupture time for stainless steel alloy decreases as temperature or stress increase. Also, it distinguished from the creep curves for the serviced stainless steel alloy, that the onset of tertiary creep stage starts early (about 50% of creep curve), this reflects the reduction in creep strength of the alloy as a result of the degradation in alloy properties that take place during the service life. It is also found that the estimated remaining working life for the serviced tube samples made from austenitic stainless steel type 321H is about (54703 hr) based on the calculated stress level (40 MPa) and service temperature 570 oC. The difference between the estimated remaining life and the experimental creep test life is due to the differences between the applied stresses, where higher level of stresses in the experimental creep testes are used to accelerate failure of specimens. The application for the result of this work can be the petroleum industries that where heaters tubes serviced at such industry need to be estimated for the remaining life after the design life expire.

Creep Resistance of S304H Austenitic Steel Processed by High-Pressure Sliding.

Sheets of coarse-grained S304H austenitic steel were processed by high-pressure sliding (HPS) at room temperature and a ultrafine-grained microstructure with a mean grain size of about 0.14 µm was prepared. The microstructure changes and creep behavior of coarse-grained and HPS-processed steel were investigated at 500–700 °C under the application of different loads. It was found that the processing of S304H steel led to a significant improvement in creep strength at 500 °C. However, a further increase in creep temperature to 600 °C and 700 °C led to the deterioration of creep behavior of HPS-processed steel. The microstructure results suggest that the creep behavior of HPS-processed steel is associated with the thermal stability of the SPD-processed microstructure. The recrystallization, grain growth, the coarsening of precipitates led to a reduction in creep strength of the HPS-processed state. It was also observed that in the HPS-processed microstructure the fast formation of σ-phase occurs. The σ-phase was already formed during slight grain coarsening at 600 °C and its formation was enhanced after recrystallization at 700 °C.

Effect of Low-Temperature Annealing on Creep Properties of AlSi10Mg Alloy Produced by Additive Manufacturing: Experiments and Modeling

The effects of postprocessing annealing at 225 °C for 2 h on the creep properties of AlSi10Mg alloy were investigated through constant load experiments carried out at 150 °C, 175 °C and 225 °C. In the range of the experimental conditions here considered, the annealing treatment resulted in an increase in minimum creep rate for a given stress. The reduction in creep strength was higher at the lowest temperature, while the effect progressively vanished as temperature increased and/or applied stress decreased. The minimum creep rate dependence on applied stress was modeled using a physically-based model which took into account the ripening of Si particles at high temperature and which had been previously applied to the as-deposited alloy. The model was successfully validated, since it gave an excellent description of the experimental data.

Assessment of microstructural evolution in cold-worked Ti-modified 14Cr–15Ni austenitic stainless steel on creep exposure

Microstructural evolution of cold-worked Ti-modified 14Cr-15Ni austenitic stainless steel on creep exposure has been studied. Creep tests were carried out at 973 K and 225 MPa on the solution annealed steel imparted with 0, 10, 20, 30 and 40 percent cold work. EBSD analysis indicated that cold working increased the fraction of low angle grain boundary resulting in increase in hardness. Creep deformation and rupture strength of the steel increased on cold working to a maximum value at around 20 percent cold work. The fraction of low angle grain boundary decreased accompanied with increase high angle grain boundary fraction on creep exposure, indicating formation of subgrain on recovery of the cold work structure. TEM investigation revealed subgrain formation as well as re-crystallization on creep exposure appreciably at higher cold work percentage, resulting in reduction in creep strength. Precipitation of fine titanium carbides pinning the dislocations was observed on creep exposure. Dislocations were found more effectively pinned by fine precipitates in 20 percent cord worked steel than that in higher cold worked steel. Deformation twins formation on creep exposure was noticed having relatively more abundant in the 20 percent cold work steel than in higher cold work, indicating more sluggish movement of dislocation in the 20 percent cold work steel. XRD investigation indicated preferential grain orientation on creep exposure with predominance presence of 220 peak over the 111 peak on cold working. Complex microstructural evolution in the cold-worked Ti-modified 14Cr-15Ni steel influenced the creep rupture strength quite differently depending on the cold work percentage.

Response of Phase Transformation Inducing Heat Treatments on Microstructure and Mechanical Properties of Reduced Activation Ferritic-Martensitic Steels of Varying Tungsten Contents

Microstructure and mechanical properties of 9Cr-W-0.06Ta Reduced Activation Ferritic-Martensitic (RAFM) steels having various tungsten contents ranging from 1 to 2 wt pct have been investigated on subjecting the steels to isothermal heat treatments for 5 minutes at temperatures ranging from 973 K to 1473 K (700 °C to 1200 °C) (below Ac1 to above Ac3) followed by oil quenching and tempering at 1033 K (760 °C) for 60 minutes. The steels possessed tempered martensite structure at all the heat-treated conditions. Prior-austenitic grain size of the steels was found to decrease on heating in the intercritical temperature range (between Ac1 and Ac3) and at temperatures just above the Ac3 followed by increase at higher heating temperatures. All the steels suffered significant reduction in hardness, tensile, and creep strength on heating in the intercritical temperature range, and the reduction was less for steel having higher tungsten content. Strength of the steels increased on heating above Ac3 and was higher for higher tungsten content. Transmission Electron Microscopy (TEM) investigations of the steels revealed coarsening of martensitic substructure and precipitates on heating in the intercritical temperature range, and the coarsening was relatively less for higher tungsten content steel, resulting in less reduction in tensile and creep strength on intercritical heating. Tensile and creep strengths of the steels at different microstructural conditions have been rationalized based on the estimated inter-barrier spacing to dislocation motion. The study revealed the uniqueness of inter-barrier spacing to dislocation motion in determining the strength of tempered martensitic steels subjected to different heat treatments.

Tensile, Fatigue, and Creep Properties of Aluminum Heat Exchanger Tube Alloys for Temperatures from 293 K to 573 K (20 °C to 300 °C)

Abstract Since automotive heat exchangers are operated at varying temperatures and under varying pressures, both static and dynamic mechanical properties should be known at different temperatures. Tubes are the most critical part of the most heat exchangers made from aluminum brazing sheet. We present tensile test, stress amplitude-fatigue life, and creep–rupture data of six AA3XXX series tube alloys after simulated brazing for temperatures ranging from 293 K to 573 K (20 °C to 300 °C). While correlations between several mechanical properties are strong, ranking of alloys according to one property cannot be safely deduced from the known ranking according to another property. The relative reduction in creep strength with increasing temperature is very similar for all six alloys, but the general trends are also strong with respect to tensile and fatigue properties; an exception is one alloy that exhibits strong Mg-Si precipitation activity during fatigue testing at elevated temperatures. Interrupted fatigue tests indicated that the crack growth time is negligible compared to the crack initiation time. Fatigue lifetimes are reduced by creep processes for temperatures above approximately 423 K (150 °C). When mechanical properties were measured at several temperatures, interpolation to other temperatures within the same temperature range was possible in most cases, using simple and well-established equations.

Effect of oxidation on the creep rupture life of ASME T23 steel

The creep rupture behavior and the effects of oxidation on the creep strength of ASME T23 steel have been investigated. At 625 °C and 650 °C, a greater degradation of long-term creep strength was observed than expected from conventional master curve descriptions in the literature. Oxide scales were observed around the surface of the crept samples tested in air at 625 °C and 650 °C but no oxide scales were observed on samples tested in helium. However, at 625 °C, the creep rupture life was almost the same both in helium and in air, indicating that factors other than oxidation are responsible for the reduction in long-term creep strength. At 650 °C, the creep rupture life was longer in helium than in air, indicating an influence of oxidation, but the long-term strength in helium was still below the master curve. Reasons for the long-term degradation of strength, including microstructural changes, are discussed.

ZIRLOTM Cladding Improvement

Abstract Improvements to fuel design have been made in recent years to meet the challenges of increases in fuel duty in terms of linear power and operating temperature. Improved cladding material is one of these design improvements. Specifically, Westinghouse has developed an improved version of ZIRLOTM called Optimized ZIRLO and denoted as OPT ZIRLO. The Sn level in this improved material is reduced from the nominal standard previous level of about 1 % to a range of 0.6 % to 0.8 %. The reduced Sn level has been optimized to produce a higher corrosion resistance and provide adequate in-reactor creep resistance. Out-reactor diameter creep tests have shown that decreasing Sn increases out-reactor creep, suggesting that decreasing Sn may also increase in-reactor creep. An in-reactor testing program in the Vogtle Unit 2 pressurized water reactor (PWR) was performed to confirm the predicted in-reactor behavior. The test samples were suspended as segmented rods inside fuel assembly thimble tubes. In-reactor diameter creep data confirmed that decreasing Sn increases in-reactor creep. As a result of the correlation between in-reactor and out-reactor creep, an extensive out-reactor diameter creep program was performed in order to develop methods to fabricate OPT ZIRLO with the same in-reactor creep properties as the currently used stress-relief annealed standard ZIRLO (denoted as SRA STD ZIRLO). The level of in-reactor diameter creep of SRA STD ZIRLO was achieved for OPT ZIRLO by two methods. One method involved changing the final microstructure from SRA to partially recrystallized (PRXA). The other method kept the final microstructure as SRA and changed the tube reduction sequence to decrease the final tube area reduction. In order to develop these methods, a series of material variation tests was performed. Some of the material variations included different final heat treatments, different tube reduction sequences, and different pre-charged hydrogen levels. These tests were performed for both out-reactor and in-reactor. In addition, the out-reactor and in-reactor creep were observed to directly correlate for OPT ZIRLO material fabricated with different final anneal temperatures. Thus, out-reactor creep may be used to predict in-reactor creep properties for different final anneal temperatures. These results show that fabrication changes may be used to control in-reactor creep. In this study, fabrication changes were used to compensate for the reduction in in-reactor creep strength associated with lower tin content in OPT ZIRLO.

Tension–compression creep asymmetry in a turbine disc superalloy: roles of internal stress and thermal ageing

The tension and compression creep behaviour of an as-received and pre-aged IN100 disc alloy have been characterised in order to validate a previous hypothesis that the unusual response of low and even negative initial creep rates in tension was caused by the presence of an internal stress field within the alloy. Absolute values of initial creep rates in compression were found to be much greater than in tension and this asymmetric creep response is conclusive proof of the presence of an internal compressive stress field in the alloy matrix. The asymmetry was virtually removed by pre-ageing prior to creep and this is attributed to the decay of the internal stress. These features have been simulated using a microstructure-based creep model incorporating an evolving internal stress field. The model also simulates the additional (and complicating) reduction in general creep strength that is thought to be due to coarsening and dissolution of the smallest particles of the tri-modal γ ′ distribution in the alloy. The net consequence of these two competing thermal processes is that the short-term creep response is dominated by the initial magnitude of the internal stress field whereas coarsening and dissolution of the smallest γ ′ particles determines the long-term behaviour.

Creep rupture and ductility of as-manufactured and service-aged nickel alloy IN617 materials and welds

This paper describes an ECCC Working Group 3C project on the creep performance of IN617 parent material and fabrication and repair welds in as-manufactured, aged, and service exposed conditions. The aims were to provide data for design of high temperature plant, to investigate the effects of service ageing, and to clarify the potential risk of relaxation cracking should an as-welded weldment enter service. A multi-member Italian consortium undertook creep testing of a new IN617 parent and weld, while Laborelec carried out parallel testing on an ex-service IN617 weld from a GT combustion system component provided by Powergen. The project also included comparison tests on the new weld after thermal pre-ageing to simulate service conditions at both moderate and higher temperatures.The results show that IN617 welds are weaker in creep than the parent material. Pre-ageing at high temperature causes a slight reduction in creep strength, but pre-ageing at a more moderate temperature actually causes a slight increase. Plain bar testing showed that IN617 weld metal can exhibit intergranular creep failure with fairly low ductility. However, notched bar testing of the ex-service material showed some evidence of notch strengthening in the weld metal and heat-affected zone, suggestive of a moderately low risk of creep brittle weld relaxation cracking in this material.

Creep deformation in a modified 9Cr-1 Mo steel

The short term creep properties and microstructural changes associated with creep were studied for constant stress specimens of a modified 9Cr-1Mo steel tested at 923, 873, and 823 K. The variation in the minimum creep rate with stress could be well described by the Norton equation (ε = Aσnexp(-Q/RT)) and many of the variations of this equation described in the literature. The values for the activation energy and stress exponent were comparable to values reported previously. The Monkman-Grant equation was found to describe well the relationship between the minimum creep rate and the time to rupture. The microstructural changes have been considered in terms of precipitates previously identified in these steels. A coarse structure (size ≤200 nm), comprising M23C6, M2X and a Nb rich MX type precipitate, was observed using SEM. A finer scale structure (size < 200 nm) was observed, using TEM, to be predominantly comprised of M23C6, M2X and a V rich MX type precipitate, with smaller amounts of M6X, V4C3, etc. After creep the coarse M23C6 and M2X precipitates were found to coarsen and the fine V rich MX type precipitates were found to increase in number. It is considered that precipitation of the V rich MX precipitates compensates for the coarsening of the M23C6 and M2X, thus offsetting the reduction in creep strength that would have been produced by the coarsening.

A Mechanistic Approach to Remanent Creep Life Assessment of Low Alloy Ferritic Components Based on Hardness Measurements

Creep damage leading to failure in low alloy ferritic steels at service operating conditions results from: (i) structural coarsening leading to a continuous reduction in creep strength during exposure, and (ii) intergranular creep cavitation. A mechanistic model is presented for describing tertiary creep which accounts for both of these factors. By assuming a limiting creep ductility corresponding to a maximum cavitation potential a lower bound life predictive approach is given. The significant structural parameters are then the background solid solution strength, the interparticle spacing (λ) and the rate of increase of λ, although the long-term, creep-rupture strength is shown to be dictated largely by the latter. Consideration is given to the determination of these kinetics by means of hardness change measurements: good correlations are found with experimental data. A methodology for characterizing the remanent creep life of component material using hardness measurements and post-exposure aging is developed on this basis.

Precipitate phases in type 321 steel

The precipitate phases in seven casts of AISI type 321 steel aged at 600°C have been studied. Apart from titanium carbides, nitrides, carbosulphides, and σ-phase, the intermetallic G-phase was also detected in samples having a small grain size. The G-phase, which has not previously been reported in type 321 steel, is a fcc nickel-titanium-silicon phase with a lattice parameter of 1·12 nm. The chemical composition was found by energy-dispersive X-ray spectroscopy in a scanning transmission electron microscope to be 63Ni–21Ti–12Si (wt-%). The formation of G-phase is inhibited by large grain size and low ‘excess’ titanium content, i.e. the amount of titanium remaining after, deducting the amounts that are combined in TiC, TiN, and Ti4C2S2. When there is no ‘excess’ titanium, the remaining carbon is precipitated as M23C6. The formation of G-phase causes a further reduction in the amount of ‘excess’ titanium in solution, and this may lead to a reduction in creep strength in long-term creep tests.MST/53

Q and mantle creep

EXPERIMENTS on model materials show that the acoustic properties of the low velocity zone (LVZ) can arise from the formation of intergranular fluid or melt with no significant reduction in creep strength. Consequently, only very limited bounds on the asthenosphere can be inferred from seismic data.